You won't feel comfortable or fashionable walking around with Max Donelan's invention strapped to your knee. The bulky 3.5-lb. (1.6 kg) gadget "is not that pleasant," says Arthur Kuo, a biomedical engineer at the University of Michigan, who co-wrote an article on the brace that appeared in Science last month. But Donelan's device pays off in other ways. Using the same principles that allow hybrid cars to recycle energy created in braking, braces worn on both knees can generate 5 watts of electricity by harvesting the energy inherent in a walker's stride. That may not sound like much, but it's enough to charge 10 cell phones, and it's absolutely free. "People like the idea of generating their own power," says Donelan, a kinesiologist at Simon Fraser University in British Columbia. "If you do things in a clever way, you can get energy cheaply."

Getting energy cheaply has never been more necessary than it is now, with oil recently breaking its all-time inflation-adjusted high price. The era of inexpensive power is over, perhaps for good, which means it's time to extend beyond energy efficiency to energy-scavenging, harnessing the sort of wasted watts we wouldn't have bothered with in the past. Fortunately, scientists are finding new ways to harvest unused energy from the environment, industrial activities and even the heat and motion of our bodies. "Energy-scavenging has been around for years, but because of the fuel crisis, everyone from big companies to small ones is looking to utilize it," says Marc Poulshock, president of Thermo Life, which produces devices that can harness thermoelectric energy. "It's a very hot topic."

One of the most abundant forms of unused energy in the environment is the vibrations that are a by-product of motion. Think of the rumblings of a bridge in heavy traffic or even the pulse of a dance floor. That's essentially free movement, and scientists can transform that micromotion into electricity in a number of ways. One should be familiar from high school physics class. A magnet hooked up to be sensitive to vibrations wobbles inside a copper coil, generating a current through electromagnetism. Steve Beeby, an engineer at the University of Southampton in Britain, created a vibration harvester that works on that principle much more efficiently than similar devices did in the past. The electricity isn't much: his devices now generate hundreds of microwatts at most, and there may be an upper limit to how much energy can really be scavenged from vibrations. "It's very unlikely on a big scale," says Beeby, who directed the European Union's Vibration Energy Scavenging project. "It will never compete with wind power or anything like that."

But vibration power does have its uses. The shaking of a bridge could power tiny sensors to monitor the structure's physical integrity. Or the steady vibrations of a beating human heart could be harvested to run a pacemaker. Not only is vibration energy free, but the power sources for devices it fuels wouldn't have to be replaced every few years--meaning cardiac patients wouldn't need their chests cut open periodically to replace the batteries in their pacemakers. "These are places where there's no source of power but plenty of vibrations," says Roy Freeland, CEO of the British vibration-power start-up Perpetuum. "You can just fit and forget."

You can scavenge motion energy more directly with piezoelectric, or electricity-sensitive, materials, which generate a charge when compressed. That's the principle behind one of the most innovative forms of energy-scavenging: rain-harvesting. Researchers led by Jean-Jacques Chaillout at France's Atomic Energy Commission found that a 25-micrometer-thick strip of piezoelectric material (the diameter of a thin strand of human hair) could produce about 1 microwatt per raindrop. That's barely noticeable, but it could be enough to power environmental sensors, especially in areas where condensation is constant--like the inside of a nuclear power plant's cooling towers. "When you add up all the materials and costs in powering, battery production and charging you save with [the strips], it really adds up," says Chaillout. A similar technology is being explored by Georgia Tech researchers who developed a piezoelectric yarn that produces a current when strands are rubbed together--perhaps giving tailors the ability to one day make a literal power suit.

But piezoelectrics pale next to the biggest opportunity to scavenge energy: heat. The thermoelectric effect--temperature differences between two ends of a circuit can be converted directly to voltage--allows us to recover some of that lost energy. For years the technology was too costly to be widely used outside extreme examples like the space program, but new companies like the California-based Thermo Life can produce energy from relatively small temperature differentials. Right now it's used mostly to power rechargeable batteries in wireless devices, but as the technology improves, it could begin to harness the vast amount of energy lost as heat in the fossil-fuel plants that provide most of our electricity. "Sixty percent of the world's energy is wasted as heat," says Rama Venkatasubramanian, a thermoelectric expert at the research firm RTI International in North Carolina. "If we could tap into just 10% of that, it would be a big thing for energy efficiency." Let's hope he's right: there's not a watt to waste.